The present invention relates to a mass flow sensor.
The mass flow sensor according to the present invention has the advantage over the related art of an improved membrane stability.
An important aspect of the present invention is that the stability of the membrane of the known mass flow sensor is increased by increasing the total layer thickness of a membrane according to the present invention in comparison with the known membrane.
One possibility for increasing the total layer thickness of the membrane and increasing the membrane stability of the known mass flow sensor is to arrange a moisture barrier above the metal layer of the known mass flow sensor.
In a preferred embodiment of the present invention, the top layer of the mass flow sensor, i.e., the membrane is designed as a moisture barrier in the form of a cover layer. In addition to increasing the total layer thickness and, as a result, improving membrane stability of the known membrane, this has the advantage that the penetration of moisture into the membrane and thus into the mass flow sensor is at least greatly reduced. In the case of uptake of moisture, there is the risk that one or more moisture absorbing layers might separate from the layer beneath them or the frame or their mechanical properties might be significantly impaired. Thus, the use of a moisture barrier according to the present invention has the advantage of further improving membrane stability in addition to the effect of shielding the membrane from moisture. Moisture can reach the cover layer or the membrane in particular through the atmospheric humidity present in the air that flows over the mass flow sensor.
In an advantageous embodiment of the present invention, the moisture barrier is formed by a nitride layer, which also improves the stability of the membrane according to the present invention with respect to particles in the air-flow striking the membrane. An LPCVD or PECVD nitride layer is preferably used as a moisture barrier.
As an alternative or in addition, however, the moisture barrier may also be formed by a silicon carbide layer, preferably PECVD silicon carbide, a layer of a chemically resistant metal such as platinum, gold, etc., or a layer of one or more metal oxides.
To further improve the mechanical stability of a membrane according to the present invention and/or to further improve the protection of the membrane from penetrating moisture, a top sandwich system having at least one oxide layer and at least one nitride layer is provided in the upper area of the membrane in the case of another preferred embodiment of the present invention. The sandwich system is preferably arranged above the metal layer of the membrane. For this purpose, silicon oxide and silicon nitride layers are preferably used.
As an alternative or in addition, another embodiment of the present invention provides for a bottom sandwich system having at least one oxide layer and at least one nitride layer to be deposited beneath the metal layer and above the frame of the mass flow sensor. Again in the case of this lower sandwich system, silicon oxide and silicon nitride layers are also preferably used.
The use of one or more sandwich systems in the membrane according to the present invention has the advantage that adequate protection from moisture penetrating into the sensor can be guaranteed even when the layer forming the top moisture layer is damaged. Due to the use of one or more sandwich system, it is also possible to adjust the membrane tension and the thermal conductivity of the membrane in a wide range through the various layers.
According to another preferred embodiment of the present invention, a CVD oxide layer, preferably a PECVD silicon oxide layer, is provided directly beneath the metal layer of the membrane. According to the present invention, the CVD oxide layer illustrated in FIG. 1 replaces the reoxide layer of the known membrane. Since the reoxide layer is produced by converting the surface of a silicon nitride layer into a silicon oxide layer, there are technical restrictions to the process with regard to the maximum layer thickness that can be produced. In the case of the known membrane illustrated in FIG. 1, only the reoxide layer is replaced by a thicker CVD or PECVD oxide layer, so it is easily possible according to the present invention to produce a thicker membrane in comparison with the known membrane.
Furthermore, according to the present invention, the known reoxide layer can also be replaced by a sandwich system, preferably composed of PECVD oxide layers and PECVD nitride layers, instead of a CVD oxide layer. In an especially preferred embodiment of the present invention, PECVD silicon oxide layer PECVD silicon nitride layers are deposited.
According to another especially preferred embodiment of the present invention, the LPCVD nitride layer between the frames and the reoxide layer of the known membrane in FIG. 1 is replaced by a PECVD nitride layer, preferably by a PECVD silicon nitride layer. Since the reoxide layer is also replaced in the known membrane, as mentioned above, it is possible to produce a membrane or a mass flow sensor according to the present invention as part of PECVD processes. PECVD processes for producing the membrane or sensor according to the present invention may typically be carried out at a lower temperature than is possible with LPCVD processes. An advantage of a low-temperature process such as the PECVD process, is that the development of oxygen precipitates in the silicon crystal and thus their negative effects with regard to dimensional accuracy in etching with potassium hydroxide (KOH) are greatly reduced.